1. Field of the Invention
The present invention relates to a novel process for the preparation of fine-particle, porous and rapidly water-swellable polysaccharide graft copolymers. These polymers are prepared by inverse suspension polymerization and crosslinking. The present invention also relates to the polysaccharide graft copolymers prepared by the present process.
2. Discussion of the Background
Water-absorbing polymers are used for a wide variety of purposes in the sanitary and hygiene sectors as water-absorbing agents in disposable diapers and paper towels, as tampons, undersheets for patients, electrolyte thickeners in dry batteries, as moisture retainers or water stores in agriculture, and as desiccants.
Suitable polymers are derivatized polysaccharides, usually grafted with water-soluble vinyl monomers, such as carboxymethyl cellulose, hydrolyzed starch/acrylonitrile graft copolymers, acrylic acid/starch graft copolymers, or completely synthetic, slightly crosslinked polymers such as partially crosslinked polyacrylic acid salts or partially crosslinked polymaleic acid derivatives.
Incorporation of starch in water-soluble graft copolymers makes it possible, compared with completely synthetic polymers, to adjust particular product properties. Thus, the porosity of the polymer particles is raised, the absorption is increased, and the biodegradability is improved.
Preparation of graft copolymers by direct grafting of starch with acrylate in aqueous solution is not straightforward industrially. Previous swelling of the starch is necessary in order to obtain the maximum homogeneity of dispersion, which is necessary for the grafting, of starch in the aqueous monomer solution. This considerably increases the viscosity of the monomer solution, and a paste-like consistency is obtained when more than about 10% starch is used.
DE-C 26 12 846 discloses the preparation of water-absorbing graft copolymers by grafting water-soluble monomers, such as acrylic acid, onto starch in the presence of a crosslinking agent. The grafting reaction is carried out in aqueous solution or in aqueous/alcoholic medium as so-called precipitation polymerization. This process results, in the case of grafting in aqueous solution, in rubber-like gels which cannot be stirred and from which final products in the form of powders are obtained only after drying and milling. By contrast, if a grafting is carried out as precipitation polymerization in the presence of an approximately 20-fold excess of alcohol as precipitant, the resulting fine-particle products have only moderate liquid absorption capacity.
According to Japanese Patent Specification 80/139 408, a graft copolymer can be prepared by polymerization of acrylonitrile in aqueous medium in the presence of starch and can subsequently be hydrolyzed and crosslinked. The result is a powder with a water absorption capacity of 150 to 180 ml/g.
Fine-particle water-absorbing polymers can be prepared by polymerization of partially neutralized acrylic acid in inverse suspension, also called reverse phase suspension. This entails a water-soluble monomer in the form of an aqueous solution being emulsified in a hydrophobic medium in the presence of a dispersant and polymerized to a fine-particle final product. In the presence of a polysaccharide such as starch, it is possible in this way to obtain water-soluble or water-insoluble, swellable graft copolymers.
A graft copolymerization in inverse suspension is indicated in Japanese Patent Specification 80/161 813. This entails initially making up a mixture of n-hexane with sorbitan monostearate, starch, water, acrylic acid, sodium hydroxide solution and water-soluble initiator, before initiating the polymerization by heating. However, in this case, the reaction product tends to aggregate during the polymerization and does not give a fine-particle product.
According to DE-C-28 40 010 it is possible to prepare water-soluble polysaccharide graft copolymers by inverse suspension polymerization, preferably in a batch process. This entails a polysaccharide being initially suspended in a solvent which is immiscible with water in the presence of a surface-active agent. Subsequently, an aqueous monomer solution which contains predominantly acrylamide or a cationic monomer, and can also have small amounts of acrylic acid, is added at room temperature. Addition of an initiator is followed by heating and polymerization. The solids contents, based on the aqueous polymerization mixture, are above 50%.
In this case, no crosslinking agents are employed and no water-swellable gel-like polymers are obtained. The indicated batchwise preparation process leads at the start of the polymerization to temperature peaks which are difficult to control in large batches.
In EP-B-0 083 022, acrylic acid is polymerized in the presence of starch in aqueous solution. The products can then be crosslinked in an inert solvent in the presence of 0.01 to 1.3 parts of water per part of resin. The preparation of the starch graft copolymer takes place at a solids content of only 20%. In addition, these starch graft copolymers - crosslinked or uncrosslinked-have only a low water absorption capacity.
According to DE-A-38 01 633, polysaccharide graft copolymers are prepared by inverse suspension polymerization, partial removal of water and crosslinking. The inverse suspension polymerization is carried out in one stage and batchwise by first mixing all the reaction components and then initiating the polymerization by heating in the presence of an initiator. In this case there is violent evolution of heat at the start of the polymerization. In industrial production reactors it is often possible only with difficulty to ensure sufficiently rapid dissipation of the heat of polymerization.
Thus, there remains a need for a process to produce fine-particle, water-swellable polysaccharide graft copolymers, which is free of the above-described drawbacks. There also remains a need for polysaccharide graft copolymers with improved particle fineness and absorptivity and liquid-retention properties.